Recently, a laser technology having 2 μm operating wavelength has been extensively studied due to a wide range of its applicability. Among others, the study on a thulium-doped optical fiber has been extensively progressed due to the capability of generating a light signal having a wavelength around 2 μm plus the excellent optical properties that the existing optical fiber lasers have.
In case of manufacturing the optical fiber laser by using the thulium-doped optical fiber, it is required to have a multi-stage optical fiber amplifier using the thulium-doped optical fiber so as to obtain a light signal having a desired magnitude. In order to realize such a thulium-doped optical fiber amplifier, a gain medium and a pumping light source are required. Here, a pumping light source having a wavelength around 800 nm or a pumping light source having a wavelength around 1,560 nm is used to generate and amplify the light signal having the wavelength around 2 μm.
FIG. 1 is a diagram showing a conventional configuration for the amplification of the low-power light signal having a wavelength around 2 μm.
In FIG. 1, an optical amplifier is realized with respect to the input light signal having a wavelength around 2 μm by employing a thulium-doped single-mode optical fiber and an erbium-doped optical fiber.
As for a high-power amplifier, it can be realized by employing a structure of a cladding pumping optical fiber using a multi-mode type of a high-power laser diode having 800 nm operating wavelength. As for the low-power amplifier amplifying a small sized signal, it can be realized by employing a structure of a core pumping optical fiber. However, such an amplifier cannot be realized due to the absence of an optical coupler associated with the high-power laser diode having 800 nm operating wavelength.
To that end, as shown in FIG. 1, an erbium laser having 1,560 nm operating wavelength with the erbium-doped optical fiber and the thulium-doped single-mode optical fiber are employed to realize a low-power core pumping type of the optical amplifier.
However, this configuration suffers from the drawback that the erbium-doped optical fiber must be used for the pumping light source, so that the configuration thereof becomes complicated and a manufacturing cost is increased.